1. Field of the Invention
The present invention relates to a scroll type compressor.
2. Description of Related Art
FIG. 3 shows an example of a conventional scroll type compressor. As shown in FIG. 3, a scroll type compressor mechanism C is arranged at the upper area inside a sealed housing 8, and an electric motor M is laid out at the lower area of this housing.
The scroll type compressor mechanism C is composed of a fixed scroll 1, an orbiting scroll 2, a rotation preventive mechanism 3, such as Oldham's coupling (link), that permits the revolution of the orbiting scroll 2 but prevents its rotation around its own axis, a frame 6 to which the fixed scroll 1 and the electric motor M are attached, an upper bearing 71 and a lower bearing 72 for supporting a rotary shaft 5, a rotation bearing 73 and a thrust bearing for supporting the orbiting scroll 2, and the like.
The fixed scroll 1 is equipped with an end plate 11 and a spiral-shaped wrap 12 erected on the internal surface of said plate 11, and supported by the frame 6 movably along the axial direction for its free movement through a spring 18.
The orbiting scroll 2 is provided with an end plate 21 and a spiral-shaped wrap 22 erected on the internal surface of said plate 21, and a drive bush 25 is rotatably fitted inside a boss 23 erected on the outer surface of said end plate 21 via a rotation bearing 73. An eccentric pin 53 protruding from the upper end of the rotary shaft 5 is rotatably fitted inside an eccentric hole provided on this drive bush 25. A balance weight 84 is mounted on the upper end of the rotary shaft 5.
The fixed scroll 1 and the orbiting scroll 2 are engaged with each other with an eccentric throw corresponding to the radius of revolution and with an angular shift of 180.degree. between them. With this engagement, a plurality of compression chambers 24 are formed with a point symmetry with respect to the center axis P of the spiral-shaped wrap 12 of the fixed scroll 1.
A discharge port 13 is provided at the center area of the end plate 11 of the fixed scroll 1, and one end of this discharge port 13 communicates with an innermost chamber 26 (formed immediately before the point where the base ends of spiral-shaped wraps 12 and 22 depart from the corresponding side spiral-shaped wraps 22 and 12, respectively).
Cylindrical bosses 46 and 47 are provided concentrically on the outer surface of the end plate 11, and the tips of these bosses 46 and 47 are slidably engaged via a seal 42 to a partition plate 41 which is fixed to the sealed housing 8 with an interposed space to the end plate 11. Thus, a high pressure chamber 44 is formed in the central area on the outside of end plate 11, and an annular back pressure chamber 45 is formed around this high pressure chamber A discharge port 13 opens to this high pressure chamber 44, while a negative pressure chamber 45 communicates gas via a through hole 19 to a compression chamber which is in the process of compression.
The orbiting scroll 2 is driven via a turning drive mechanism, such as the rotary shaft 5, an eccentric pin 53, a dry bush 25, a boss 23 and the like by the electric motor M, whereas the orbiting scroll 2 makes a revolution motion on a circular orbit with a revolution turning radius while the rotation around its own axis is prevented by the rotation preventive mechanism 3.
Then, the gas enters into the sealed housing 8 through a suction pipe 82, and after cooling down the electric motor M, it passes through a channel 85 provided on the frame 6 and also through a suction chamber 16 from a suction channel 15 and is sucked into the compression chambers 24 from the external end openings of the spiral-shaped wraps 12 and 22. The gas reaches an innermost chamber 26 located in the central area while it is compressed as the volume of the compression chamber 24 decreases due to the revolution of the orbiting scroll 2. It then passes through the discharge port 13 to discharge into the high pressure chamber 44, and enters into a discharge cavity 48 through a hole 43 provided on the partition plate 41, and is finally discharged to the outside via a discharge pipe 83.
At the same time, lubricating oil 81 which is stored at the inner bottom of the housing 8 is sucked up by a centrifugal pump 51 installed in a lower portion inside the rotary shaft 5, and after lubricating the lower bearing 72, the eccentric pin 53, the upper bearing 71, the rotation preventive mechanism 3, the rotation bearing 73, the thrust bearing 74, and the like through an oiling port 52, it returns to the bottom of the sealed housing 8 via a chamber 61 and an oil discharge port 62, and is stored therein.
Further, because the discharged gas under high pressure is introduced into the high pressure chamber 44 under the revolution motion of the orbiting scroll 2 and the medium pressure gas in the process of compression is introduced into the back pressure chamber 45, the end plate 11 is pressed downward by the gas pressures inside the high pressure chamber 44 and back pressure chamber 45. The tip surfaces of spiral-shaped wraps 12 and 22 are pressed with an adequate contact pressure against the internal surfaces of end plates 21 and 11, so as to maintain each of a plurality of compression chambers 24 in sealed conditions.
Also, the high pressure chamber 44 and back pressure chamber 45 are formed concentrically with respect to the center axis P of the spiral-shaped wrap 12 as a center. This is because, if the center of urging pressure forces acting on the end plate 11 due to gas pressures do not coincide with the center axis P of the spiral-shaped wrap 12, an overturning moment occurs which prevents the tip surfaces of the spiral-shaped wraps 12 and 22 from being pressed with a uniform contact pressure against the internal surfaces of the end plate 21 and 11, thereby causing the defective sealing of the compression chambers 24.
This conventional scroll type compressor makes an adequate pressing force acting on the end plate 11 by appropriately setting the pressure receiving areas of the high pressure chamber 44 and back pressure chamber 45, but in order to decrease fluctuations of the pressuring forces which accompany pressure changes in the compression chamber 24 to a minimum level, the pressure receiving area of the high pressure chamber 44 should preferably be made smaller than that of the back pressure chamber 45. In other words, it is preferred that the area ratio of the high pressure chamber 44 be made smaller.
However, because the discharge port 13 is provided at a position shifted sideways from the center axis P of the spiral-shaped wrap 12 and the pressure receiving area of the high pressure chamber 44 is set to a large size so as to include this discharge port 13, the area ratio of the high pressure chamber 44 is large and the pressing force acting on the end plate 11 fluctuates greatly. As a result, if the pressing force becomes too small, the sealing of the compression chambers 24 becomes insufficient. On the other hand, if the pressing force becomes excessive, frictional forces between the tip surfaces of the spiral-shaped wraps 12 and 22 and the internal surfaces of the end plates 21 and 11 increases, thereby causing trouble such as power loss of the compressor.